In large audio systems such as those used in schools, auditoriums, airports, hospitals, and athletic stadiums, the electrical energy to drive speakers is distributed much in the same fashion as ordinary electric power. The output voltage from the power amplifier although the amplitude will vary is considered or assumed to be a constant voltage similar to common 120 volt AC lines which we find in our homes except that the frequency is varied in such audio systems. Many amplifiers used for this purpose have constant-voltage output taps for the industry standardized values of 25, 70, and perhaps 140 volts, as well as the standard impedance taps. The voltage value of the amplifier, and hence the system, is the RMS output voltage of the amplifier when it is driven to full power by a sine wave, and with a load connected to the amplifier. The constant-voltage taps correspond to impedance taps, typically at the output transformer secondary, by: ##EQU1## It can therefore be seen that, for example, for a 25 volt system and a 10 watt amplifier the required constant voltage tap would occur at 62.5 ohms, given the same situation but with a 100 watt amplifier, the same 25 volt tap would be placed at 6.25 ohms.
Amplifiers used in constant-voltage systems generally incorporate substantial negative-feedback to insure that variations in load will have virtually no effect on the output voltage of the amplifier. The speakers are designed to accept this nominally constant voltage from the amplifier and differ from each other essentially only in power consumed, such as a 100-watt and a 300-wattlight bulb do, as the electric power analogy is again used. There are several important advantages to this method of audio distribution. First, by using a relatively high line voltage, power loss in the wire between the amplifier and the load is minimized, even when using wires of relatively small size. For example, while an 8-ohm speaker can only be 70 feet from the amlifier to suffer a 10% power loss with a connecting cable size of #18 AWG, a 1-watt speaker in a 70 V system can be over 17,000 feet from the amplifier with the same loss with a wire size of #22 AWG; 2 1/2 times smaller than #18. Calculations are simplified in the constant-voltage system also. Instead of being required to perform a series of tedious, involved calculations to determine impedances in numerous series and parallel branches, or trying to devise a hookup given various speaker impedances and sound level requirements to match a particular output impedance tap on an amplifier, one may simply parallel speakers as necessary and add wattages of individual speakers to obtain the total power of the system, in watts.
In practice, the speaker load consists of one or more speakertransformer combinations. The speaker is of a conventional design, with conventional voice coil impedance; usually 4, 8 or 16 ohms. The transformer is designed to work into this impedance and the primary side of the transformer accepts the line level voltage - 25, 70, or 140 volts. The primary usually has taps, and these are calibrated in watts, the ratings of which are only valid for a system voltage for which the transformer is designed. For other system voltages, the transformer will work, but at a proportionately differenct power level.
Even though determination of total power utilized in a system is simplified for the system designer, an actual installation may present difficulties for the installer or the serviceman called to install a system or repair an existing installation which may have been modified by unskilled personnel.
Occasionally a transformer will develop a short, for example, if a mounting screw penetrates the windings. A voice coil may open. Perhaps a transformer is connected backwards, to the wrong tap, or is not connected at all. One leg of the distribution line may short to ground and the other leg also short to ground in a totally different part in the system; taken together they will short out the amplifier. A speaker with the wrong voice coil impedance or a transformer designed for the wrong voltage may be used. These potential problems are multiplied by the use of several hundred speakers in a large installation. There are even occasions where the system voltage is unknown.
In the past, servicemen have relied on DC resistance checks with a conventional ohmmeter, virtually worthless in a system designed to transmit audio signals. DC resistance from the line and transformer primaries may vary widely and are so close to zero any degree of accuracy is totally nonexistant. Some servicemen have also used impedance bridges which are time-consuming to null, and then converting the impedance found into power consumed given a constant voltage. Again very time consuming--especially in view of the many circuits which must be checked. This instrument described may be used in place of the central power amplifier, across individual speaker lines, tracing problem lines throughout the complex all the way to the speaker itself, measuring power consumption of the connected load. This unit needs no nulling and reads out directly in watts, and also features impedance scales which may be used regardless of whether the sound system is of the constant-voltage type or of the more familiar form of parallel or series-parallel speaker systems.
The unit may likewise be used to check how various network's input impedance varies with frequency when used with the external oscillator, such as filters, pads, attenuators, crossover networks, speakers, etc. The feature of no nulling permits a very fast analysis.
The low voltage used for testing the system permits analysis of the system under normal circumstances, that is, with people present -- school occupied, with people seated in an auditorium, etc. There is no need to run a full-voltage signal for a complete check.